LEDs ♦ news digest
Inexpensive Transistors for Controlling High Efficiency Systems (SWITCHES).” Both FOAs seek to fund innovative WBG semiconductor materials, device architectures, and device fabrication processes that promise to enable increased energy density, increased switching frequencies, enhanced temperature control, and reduced power losses in a range of power electronics applications, including high-power electric motor drives and automotive traction drive inverters.
Traditionally, silicon devices dominated the power device market. A graph showing silicon power device prices is shown below.
two key limitations. Firstly, substantial gate/drain lateral spacing must be maintained to allow for high breakdown voltages. This requirement substantially reduces the effective current density (relative to die size) that can be achieved in these devices and also leads to a reduction in effective current density as breakdown voltage is increased. Low current densities drive down the number of die that can be fabricated on each wafer as voltage ratings increase, thus increasing the cost for a given amperage rating.
Secondly, thermal management is complicated by the fact that all current flow is confined to a relatively thin portion of the device near the top surface. Joule heating related to device losses must be dissipated across the thickness of the substrate, motivating research into advanced wafer thinning or complicated thermal spreading approaches to device assembly.
Graph Illustrating Silicon Power Device Prices According to Device Type (Credit:ARPA-E)
But as this material has several important limitations, silicon is now having to compete with wide bandgap semiconductors in the form of SiC and GaN.
SiC can operate at a higher temperature (up to 400°C) and has a lower thermal resistance than silicon, allowing for better cooling.
Strengths of GaN include the promise of making devices with incredibly low loss, and the opportunity to deposit epilayers on standard silicon substrates. The latter virtue enables production costs to be significantly below those for SiC.
The dominant GaN device architecture today is the High Electron Mobility Transistor (HEMT) heterostructure, which is depicted in Figure 1(a).
In contrast, vertical GaN device architectures as illustrated in Figure 1(b), could overcome these limitations. Such device architectures for GaN power semiconductor transistors, could substantially reduce cost and increase current densities (relative to die size).
Vertical device structures for GaN have, thus far, received relatively little attention in the research community but have been recognised as a necessary eventual device architecture for use in high power automotive applications. As with vertical FET and IGBT technologies in silicon, it is expected that vertical devices will be able to achieve higher effective current densities and will enable improved thermal management. Recent demonstrations of high-voltage vertical structure GaN devices appear very promising.
However, although GaN and SiC are set to lead the way in the power semiconductor market, ARPA-E will consider all proposals that show strong evidence of being able to meet or exceed all of the FOA targets. These include those focused on fundamentally new approaches to silicon-based devices. And applications will not be excluded solely based on the selected semiconductor material.
While all technology-focused applied research will be considered, two instances are especially fruitful for the creation of transformational technologies. The first is establishment of a technology based upon recently elucidated scientific principles. The second is the synthesis of scientific principles drawn from disparate fields that do not typically intersect.
1(a) Dominant GaN device architecture today, the HEMT heterostructure 1(b) Vertical GaN device architecture (Credit:ARPA-E)
However, the lateral GaN HEMT device architecture has July 2013
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